1. Field of the Invention
This invention relates to gas springs, and more particularly to a safety clamp device for attachment to a gas spring in order to provide additional resistance to movement of the piston rod relative to the cylinder to compensate for loss of gas pressure that would casue the spring to lose its effectiveness.
2. Description of the Related Art
Gas springs generally include a closed end cylinder containing gas under pressure, a piston movable within the cylinder and against which the gas pressure acts, and a piston rod connected to the piston and extending outwardly from the cylinder at the end opposite from the closed end. Such gas springs are used in a number of applications to serve as a counterbalancing device to counterbalance the weight of a movable member to permit the movable member to be held in a particular position relative to a stationary member. For example, gas springs are often used on automobiles to serve as counterbalancing devices for trunk lids or hatch covers, particularly on so-called hatchback models. The closed cylinder ends of the gas spring is pivotably attached to either a part of the frame surrounding the trunk or hatch opening or to a part of the trunk lid or hatch, and the free end of the piston rod is pivotably attached to the other part. When the lid or hatch is closed the piston rod is retracted into the cylinder, and causes an increase in the pressure of the gas, thereby tending to urge the lid or hatch into an open condition. When the lid or hatch is unlatched, the gas pressure in the gas spring aids in overcoming the weight of the lid or hatch and makes it easier to open. When the lid or hatch is fully opened, the gas and the cylinder is at a sufficient pressure to hold it in an opened condition, yet is not so high as to result in excessive resistance when it is desired that the lid or hatch be closed.
Under certain conditions, the effectiveness of such a gas spring is reduced to the point that it becomes more difficult to lift the hatch from a closed to an open position, and the hatch no longer remains in the fully opened position. That condition results from wear that takes place in the gas spring over a period of time as a result of frequent usage, which permits the gas in the spring to slowly escape, thereby reducing the gas pressure and the effectiveness of the spring. Another condition that could cause excesive force to be required to open a hatch occurs when the temperature of the gas in the spring falls to a low level, with a corresponding reduction of the gas pressure, for example in cold climates.
When the pressure in the gas spring has fallen to such a level that the effectiveness of the spring is reduced to the point that it no longer is able to hold the hatch in an opened condition, the normal reaction of a user is to discard the entire spring and to obtain and install a new one. However, such springs are relatively expensive devices, and it is therefore desirable to attempt to find an alternative solution that would not require the purchase of a new spring.
One way in which a worn gas spring can continue to be used so that it can maintain a lid in an open condition is by means of a stop device, such as that shown in U.S. Pat. No. 3,566,435, which issued Mar. 2, 1971, to Kazuo Nakamura. The stop device disclosed in that patent is a coil spring member that is loosely carried on and slides along the piston rod, and that includes a leg that extends from the coiled portion of the structure to contact the end wall of the cylinder adjacent to the point at which the piston rod passes through the cylinder end wall. The stop device hold the piston rod securely relative to the cylinder by means of a torque that is applied by the leg to the coiled portion, which then rests against the grips the rod. However, that device is inconvenient to use because it requires additional manipulation to set it in position each time the hatch is opened and to release it each time the hatch is closed. Furthermore, because the device is loosely carried on the rod when the piston rod is retracted into the cylinder it can cause undesirable rattling noise if applied to an automobile hatch spring when the automobile travels over bumpy road surfaces.
Another solution to maintaining a hatch cover in an opened condition is disclosed n U.S. Pat. No. 4,634,170, which issued Jan. 6, 1987, to John S. Lach. That patent discloses a device that is intended to be placed between the end of the cylinder through which the piston rod passes and the point to which the free end of the piston rod is attached to the lid structure. Basically, that device serves as a block or bar to prevent the weight of the hatch lid from pushing the piston rod into the cylinder of a worn gas spring. That device is inconvenient to use because it must be separately carried in the vehicle and then placed in position and subsequently removed from position. Because it is a separate part it is possible that it could easily be lost or misplaced and therefore unavailable when needed.
It is therefore an object of the present invention to overcome the shortcomings in the existing arrangements described above, and to provide a self-contained safety clamp that is carried on a gas spring to permit the piston rod to be conveniently maintained at any position relative to the cylinder. Summary of the Invention
Briefly stated, in accordance with one aspect of the present invention, a safety clamp is provided for a gas spring in order to control the position of a piston rod relative to a gas cylinder. The clamp includes attaching means for attaching the clamp to a fluid-containing cylinder that carries a longitudinally movable piston and piston rod. The clamp also includes rod clamping means connected with the attaching means for clamping and frictionally engaging the surface of the piston rod in order to selectively control the rate of longitudinal movement of the piston rod relative to the cylinder.